Magnetic coupling pumps

The author: Dr. Thomas Herbers Technical Director, Klaus Union

If we consider the entire pump unit – from the drive motor through the elastic coupling to the pump – the potential energy savings vary. The minimum efficiency levels to be achieved by asynchronous motors are defined in IEC 60034-30 by energy efficiency classes IE1 to IE3 and are between 70 % and 96 % at nominal load, depending on the size. If load operation is below 50 % of the nominal load, the efficiency worsens decidedly. When selecting drive motors, you should therefore pay attention to the energy efficiency class and choose a model which is preferably not operated at less than 50 % of nominal load in the part-load range.

With magnetic coupling pumps a lot of energy can be saved as a result of the magnetic drive. Until now, this pump design has had an important drawback compared to pumps with a face seal or gland packing: energy is lost in the metallic isolation shells of the magnetic couplings due to the eddy currents which occur du-ring operation. Using non-metallic isolation shells prevents these eddy current losses and thus significantly increases the efficiency of a magnetic coupling pump. A typical example shows which savings are possible. Conventional magnetic couplings have an efficiency of between 90 % and 97 %. With the pump sizes typically used in the chemical industry, which have an output of, say, 10 kW at 2900 min-1, 90 % efficiency can be expected from the magnetic coupling, i .e. around 1 kW is lost there. Assuming that these pumps are working around the clock, this adds up to a potential saving of about 8800 kWh per annum.

Klaus Union started using non-metallic isolation shells, particularly made of advanced ceramics and plastic, for all kinds of applications many years ago. Today, standard solutions with ceramic isolation shells are limited to 25 bar system pressure and 250 °C with a transfer output from the magnetic coupling of around 140 kW at 2900 min-1. Pumps with plastic shells are currently used at up to 16 bar and 120 °C, the maximum transfer output being around 300 kW.

The specification and operation of the pump units are also important when it comes to saving energy in addition to choosing energy-efficient pumps and drives. When the operating points are defined at the planning stage, the safety factors are often too high, with the result that the pump units no longer operate efficiently at part load. They must be regulated down to their operating point. In practice, this is generally achieved by throttling the flow rate. This method involves low initial investment costs and is also user-friendly. Compared to other regulation options, however – especially regulating the motor speed – throttle regulation seems to cause considerable extra expense when the pump is running. The respective energy savings that can be realised with motor speed and throttle regulation obviously depend to a large extent on the individual circumstances in which the pump is used and must be considered on a case-to-case basis, though practical experience has shown that savings of 50 % or more are altogether feasible.

A good deal of energy can also be saved if the minimum volume control function of the pumps is optimised. Centrifugal pumps must not be operated against closed gate valves on the discharge side. To prevent mechanical damage or overheating, the minimum flow rate of a centrifugal pump should be around 10 to 30 % of the flow rate for the best efficiency level. Bypass pipes are often installed on the system side for this reason. The energy costs arising from the constant bypass flow are very high. One possible alternative might be to fit minimum flow valves to the pump’s discharge side. These valves open or close the bypass line automatically without any auxiliary power, depending on the flow rate and the amount to be pumped.

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